There were some questions posted as to trim drag of aircraft. It is common practice on the large commercial jets to move fuel both fore and aft to reduce trim drag. I believe for instance, the 747 has a rather large tank in the tail for just this purpose.

In terms of static stability and speed of the aircraft, these do change with aircraft speed. An airplane can be quite a bit more tail heavy at the upper end of it’s flight speed, the problem is slowing for landing. Airplanes such as a P-38 were able to use this, since their gears retracted rearward.

Today, with the electronic stability augmentation systems like on the F-16, the airplane can be flown in what would be considered unstable. Interesting to note, that the F-16 C increased the size of the tailplane by about 10% so that the bombing load could be increased in total weight. If memory serves me correctly, I believe that they were able to add two additional 500 lb. bombs with this change due to the tail carrying more of the load. In a sense, aircraft flown in this configuration are canards flying backwards.

On model pylon racers, most have difficulty just meeting weight with the required equipment, however, I have seen one that had instrumentation onboard to measure airspeed. It would not be too difficult to measure G loading. Perhaps more important would be taking data to determine what flight path gives the fastest times. A larger radius turn results in a lower loss of speed due to lower G loads, but results in a longer path. The trade-off here is how well does the airplane accelerate after the turn. Acceleration is influenced by the propeller selection.

Bill, thanks for explaining the "aft CG is faster" premise so clearly and concisely.

As for why I was moving heavy cargo, it was a passenger who needed a bit of privacy for a few moments. Upon realizing that the extra speed would get us to our destination nearly 45 minutes ahead of schedule, and he was in no way interested in taking the controls, we decided that he should stay in the back seat for the duration of cruise. He came back up front prior to descent.

I used to build engine powered freeflight models that balanced on the trailing edge of the wing, but they had stabs with roughly 50% of the wing area. The important thing is to keep the CG slightly ahead of the center of pressure of the entire airplane, and those old freeflights had their center of pressure behind the wing trailing edge.

A hypothetical model with zero mass would probably be unstable, since the air influenced by the airplane has mass, and moves with the airplane. I have built models with wing loadings of well under one ounce per square foot, and their CG location is just as important as with far heavier wing loadings, and seems to like to be a little further forward than that of the heavier jobs. The mass of air influenced by a wing has a significant influence on flutter calculations. Flutter analysts frequently consider the mass of air influenced by a wing or stab to be roughly equal to that of a cylinder of air with radius equal to the wing chord.

This entire debate that is going on in this thread has been quite humorous... It can be summed up in a VERY short saying...
Best Overall Lift/Drag ratio of plane = best speed Period...

As for Cg / NP Placement on low Wing Loading All you are doing is changing a "normal" plane into a canard and vice versa depending on whos STABILITY criteria you want to meet.

BFoote

Normal-----

A normal F16?
A normal Cessna 172?
A normal Flying Flea?
The application really defines the setup -
I just did two nice .40 powered kits for an article - very basic sport designs - CG's aprox 25%of MAC
very easy to fly . Not normal models for my own interest.
A given definition of "normal"-can be a bit of a canard also.
It is certainly interesting to see the various ideas as to what will an will not fly-- especially when the "proof " is referrenced to some text book.
Some formulas should read, "open book and close mind".
(present company excepted)

"Best overall Lift/Drag ratio" of a plane has nothing to do with "best speed". For a given configuration, there is only one angle of attack for maximum L/D. Suppose for a second that an airplane was at its angle of attack for maximum L/D when also at its maximum straight and level airspeed. How would this airplane have reached maximum level speed? At ANY airspeed less than maximum, the airplane would have more drag than thrust. It would be hard for this airplane to accelerate toward maximum level speed while decelerating.

The point I am trying to make (and others have made it here also) is that best L/D isn't really important for top speed. Airplanes designed for high speed (fighters and racers) don't tend to look like airplanes designed for high L/D (gliders), and not just for structural reasons.

How true - some of the fastest planes (closed course) had little stubby wings.
Even the "best" German fighter -prior to WW11 - was a cobbled up thing with small wings - and acheived the winning speed with an engine designed to do just that - go like hell for the contest.
Too many, equate the "best " airplane design with thermal glider design.
It keeps em happy -----

Best L/D of the airplane configureation, since you guys were talking trim drag and moving "cargo" around to increase speed, I am not talking overall design of an airplane here, that is an entirely different set of requirments and criteria to be met with a set amount of thrust.

Reason F-16 uses its horizontal stabilizer to fly on is because of the huge parasite drag at high airspeeds, thus if you balanced the fighter the same as say a Cessna 172, you would have the horizontal stabilizer doing nothing but creating Parasite drag and a bit of Induced drag etc. Take that same horizontal stabilizer, load it up, decreasing the load on the main wing which decreases its needed angle of attack decreasing induced drag and pressure drag at high speeds creating an overall best L/D ratio of the entire plane with set amount of thrust from the engine.

In my previous post I was not talking about the generic soaring definition of best L/D which is what you are refering to. Best L/D of a powered airplane at said thrust is completely different because you are burning a set amount of fuel creating x amount of thrust, and the overall best L/D with said thrust will create highest airpseed.

Brian
was on my brothers account sorry...

While reducing drag by shifting CG will increase top speed, I really don't think that's the reason for the F-16's CG location. The neutral point of an aircraft shifts aft when supersonic. If the neutral point shifts aft but the CG does not, static pitch stability increases. An increase in pitch stability reduces pitch authority. To preserve supersonic maneuverability, the CG of the F-16 is located such that stabilators lift upward when in straight and level subsonic flight (and the airplane is statically unstable in pitch). An F-16 is operating nowhere near its L/D max when at its highest speed.

Watch pictures of the Russian fighters which will do the "cobra" maneuver and do -literally flip flops including stuff where the craft moves backward- all with pilot -NOT computer inputs.
The F16 as well as many new designs are made to require very low control surface effort- (did I say that right?)
the "normal" setups" so often referred to here - would be abnormal on this type craft.
I wa truly amazed at earlier inputs on this thread which were adamant on fixed (fixated)CG setups for "correct and normal" airframes -----

Shoe: I agree wrote it a slightly diff way. Why we are even debating compressible fluid flow questions in this forum I don't know. Fighter aircraft usually operate around 50% MAC when going to supersonic conditions because lift is generated via a different pressure differential.
Mach >1 pressure gradients on the top of the wing are fairly even while in M<1 pressure gradients are highest at leading edge. Thus the Cg moves when going through the transonic range to supersonic.

Brian

The CG moves where you put it, regardless of airspeed. The Center of Pressure (CP) and Neutral Point (NP) move aft as you pass through M=1.

Best speed for a given thrust is with the lowest drag. I don't care what the lift to drag ratio is if I got the required lift and the lowest drag. Maybe if I was flying to the maximum altitude it would become a much larger concern.

Flying unloaded (1g) you might make the argument for a short wing span, but once you have to turn to stay on a closed course, then wing span (or rather Span Loading) become much more important. Since you want the low drag in the turns as well as the straight-a-way. That why Formula One racers have increased the wing spans from the early days to the present. The main limitation is weight, since it take more material to build a long wing.

Aircraft design is a bit like rock-paper-scissors!

Interesting bit - the original layouts were for Goodyear Racing - and other closed course stuff - for spectators.
I still have a full page of planforms of all the Goodyear racers from 1949- you can see and compare at a glance, the various setups --mostly short span.
Bear in mind - these were typically budget designs - using plywood tubing and cloth--
Manytimes, design changes are based on today's technologies ; composites, high strength metals etc...
For modelers - the early setups are designed like a model
Perfect!
My Cassut -- tho I am doing a 44% scale of the 15' version -- also could be done with very thin long wing as used on the newer racers flown at Reno.
The short span model can be turned extremely tight with no fear of snapping out.
A race plane -as you note - has to match the task---
Mine will be an aerobatic setup.
I have already played with various planforms - the stubby setup is excellent for my use.

LOL, I just reread what I wrote about the Cg moving... haha, sorry for muddying the waters there. You are right shoe thanks for fixing my goof. Its the Cp that moves not the Cg.

Brian

When I do use a CG --I tape it solidly in place .

Haha... I think this is great... humor has finally returned.

I think my "Cg" moves cuz in my airplanes I got little aliens dancing around in the cockpit!

cheers

Brian

When I posted the link on September 20th last year, I never thought for a moment that it would still be generating such interest almost a year later; over 7200 hits and 217 posts!! Not many threads survive that long or generate that much interest or that many responses. I envisaged a few newbies throwing in an occasional comment after visiting and picking up a few pointers.

I see comment, and even argument on national heroes, until it's lightened up with a little humour.

Apart from a second post on the same day, I think this is my first look back here, and many of the posts surprise me; some more than others. I've long ago forgotten much aerodynamic theory, but I practise it almost every day, and it has to be one of things in life that remains true to the theory that I learnt so long ago. As a full scale jet transport pilot, I know that the principles established at Kittyhawk a hundred years ago remain as valid today as they were then. I think much of what Wilbur and Orville achieved was a mystery to them; it was accepted because it worked and they had no theory to back it, but successive generations of designers have honed the science of aerodynamics to what we know and accept today.

We can have opinions; we can fly creations that seem to defy the science of aerodynamics, but I believe that when we think we are defying the 'laws', we are missing relevant points just as the Wright brothers did back at Kittyhawk. The 'laws' of science and aerodynamics will always apply whether we believe it or not.

I agree with dick hanson though, when he says that 'more people should be into this weird stuff!!' but are we thinking about the same things dick?

I'll take another look in next year.

yep without shifting some mass, the CG doesnt move, well it does as fuel is consumed, but its the CP that moves forward with increased AoA

Unless you take the theory to extreme ends - you are simply "stuck in the middle".
Look at the arguments on flat wings -
Lo and behold - they are efficient!
(provided you know when and where to use them)
My hero for knowing what the hell is really going on is/was Alexander Lippish.
(Who?)

Alexander Lippish the Genius behind the Me-163------among other things!

In the late 40's I flew an Orbit CL with a McCoy .60. I then developed "Franks First Law".

"With enough power, you can fly a brick"

OK..OK..!! FIRST, I FEEL DICK HANSON IS 100 % CORRECT..ALL OBJECTS HAVE A C/G.. SOME FLY, SOME ,DO NOT.. I AM AND OLD SCHOOL DUDE.. IF THE RESULTS DON'T MATCH THE DATA,GET NEW DATA!! DON'T ARGUE WITH RESULTS..COMPUTERS AND FORMULAS ARE COOL, BUT,
NOT THE LAST WORD.. IF THEY WERE,WE WOULD NOT NEED WIND TUNNELS OR TEST PILOTS.. LIGHT PLANES DO FLY BETTER..HAVE YOU EVER BUILT A HEAVY PLANE INTENTIONALLY?? HEAVY PLANES, ZERO MISTAKES. NO TAKE OFF, GROUND LOOPS,TAKE OFF SNAPS,STALLS,SNAPS IN HI A.O.A, PITCH CHANGE WITH SPEED,FLYING WITH THE TAIL DOWN,LANDS TOO FAST,SNAPS WHEN SLOWED DOWN,LONG GROUND RUNS TO TAKE OFF,, ..LOTS OF TROUBLES, CAUSE ,MOST LIKELY, HEAVY!!! FORMULAS AND COMP. HAVE TAKEN SOME OF THE, COMMON SENSE THINKING ,OUT OF THIS AREA..TLAR WORKS PRETTY DARN GOOD.. SOME DESIGNERS TRY TO MAKE SIMPLE THINGS INTO ,BLACK MAGIC..BEST KEPT SECRETS,, SOOO------ THEY CAN HAVE A JOB NEXT YEAR!!! WE FLY ALL THE LATEST JETS ,F-111,F-14,,F-16,,F-22,F-18,WITH,,,,,,, NO COMPUTERS!! GO FIGURE.. THROW A STONE INTO A PACK OF DOGS,THE ONE THAT YELLS, IS THE ONE YOU HIT!!!

Hi everyone. this is my first post. My head is still spinning from reading all of your posts in this topic. I came to the aerodynamic thread to find out more information about dihedral.
I am scratch building my first plane soon and I'm concerned about when and why you use dihedral. I was told that dihedral is important on a low wing aircraft for stability and it is very important for planes that have rudder only steering. so if I'm building a low wing aircraft with ailerons/flaperons do I need dihedral? and if I do need it, is there a formula for the angle?
thank you,
crankypants